Move NotFoundExc from 'backend' to a new 'exc' module.

cc @ardumont @anlambert

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Migrated from D3128 (view on Phabricator)

yes (that's similar as what storage does for example)

thanks.

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Rebasing onto 4e19854e...

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commit 6cb7a671b5cc029ea42909874adae0aa85174973
Author: Valentin Lorentz <vlorentz@softwareheritage.org>
Date:   Tue May 5 17:53:22 2020 +0200

    Move NotFoundExc from 'backend' to a new 'exc' module.

See https://jenkins.softwareheritage.org/job/DVAU/job/tests-on-diff/7/ for more details.

Merge request was merged

closed

tagged v0.0.33